It is useful to measure the received optical power in fiber optic links in order to determine the integrity of the signal path and terminal devices. The photo-current in the photo-diode is proportional to the received power. Hence, one way of measuring received optical power is to measure the photo-current level.
Typical optoelectronic receivers utilize a photo-detector which is integrated in the same TO (Transistor Outline) package as the pre-amplifier. Most TO packages have only four pins. Two of the pins are connected to receive a differential signal from the pre-amplifier, one of the pins receives a power supply voltage, and one of the pins is for ground. Since all four pins of the TO packages each have a defined function, there is no separate lead available to output the photo-current. A fifth pin would not be an ideal solution because space is at a premium in TO packages.
Several prior art signal detectors measure the photo-current level without the use of a fifth pin. At low signal amplitudes (e.g., at 0.5 mV or less), however, these signal detectors exhibit a significant loss of gain—and thus accuracy. This loss of gain at low signal amplitudes is caused by a breakdown of the switch-like behavior of these signal detectors.